1. Field of the Invention
The present invention relates to a support for an imaging material, more specifically to a support whose surface on one side of a paper substrate composed mainly of a natural pulp where an image-forming layer is to be formed is coated with a resin sheet, which support not only can provide an imaging material and a print thereon having a high visual gloss and being free of non-uniformity in gloss, particularly silver halide photographic paper and a print thereon (silver halide photographic paper print will be sometimes abbreviated as “photographic print” hereinafter), but also is improved in the property of peeling from a cooling roll used when the support is produced so that no non-uniformity in peeling occurs, and further, which support has excellent curl resistance and an excellent stiffness and can be stably produced at a high speed.
2. Explanation of Related Art
Generally, an imaging material is constituted of a support for the imaging material and an image-forming layer provided on the support. For example, a silver halide photographic material, an inkjet recording material, a thermal diffusion transfer type heat transfer record receiving material, a heat-sensitive recording material or a photosensitive-thermosensitive recording material is produced by forming an image-forming layer such as a silver halide photograph constituting layer, an ink receiving layer, a thermal transfer type heat transfer record receiving layer, a heat-sensitive color-forming layer or a photosensitive-thermosensitive color-forming layer on a support for an imaging material, respectively, and optionally forming an undercoat layer, a protective layer, and the like. In particular, a silver halide photograph constituting layer is constituted of a silver halide photograph emulsion layer, a protective layer, an undercoat layer, either an intermediate layer or a color mixing prevention layer, either a halation prevention layer or a filter layer and an ultraviolet absorbent layer, or a combination of some of these. For example, a simple silver halide photographic material is structured by forming a silver halide emulsion layer and its protective layer on a support for a photographic material. Further, a multi-layered silver halide color photographic material is structured by consecutively forming silver halide color photograph constituting layers such as an under coat layer, a blue-sensitive silver halide emulsion layer and an intermediate layer, a green-sensitive silver halide emulsion layer and an ultraviolet absorbent layer, and a speed-sensitive silver halide emulsion layer and a protective layer, and the like on a support for a photographic material.
There is conventionally well known a resin-coated paper support in which the surface of a base paper for a support for an imaging material is coated with a resin having film formability. Concerning a support for a photographic material for use in a silver halide photographic material, for example, there is known a support for a photographic material in which a base paper is coated with a resin having film formability, preferably a polyolefin resin. There is also known a support for a photographic material in which both surfaces of a base paper are coated with a polyolefin resin. Further, after the application of the rapid photographic development treatment method of a silver halide photographic material, a support for a photographic material in which the both surfaces of a base paper are coated with a polyethylene resin is mainly practically used as a photographic paper, and the resin layer on one side where an image-forming layer is formed generally contains a titanium dioxide pigment for imparting sharpness as required.
Further, there is known a thermal transfer record receiving element having, as a support, a resin-coated paper of which the resin coating has a surface roughness of 7.5 microinches-AA or less, particularly, a polyethylene-resin-coated paper of which the base paper is surface-coated with a polyethylene resin. There is also known an inkjet recording sheet having a resin-coated paper as a support.
However, a resin-coated paper type support for an imaging material, i.e., a support which is formed of a base paper, particularly a base paper composed mainly of a natural pulp, and which is surface-coated with a resin layer on a surface side where an image-forming layer is to be formed, still has several serious problems, and actually, no satisfactory achievement has been obtained.
First, in a resin-coated paper for use as a support for an imaging material, a base paper is coated with a resin having at least film formability, particularly a resin layer containing a polyethylene-based resin, on a surface side where an image-forming layer is to be formed (a surface on which an image-forming layer is to be formed will be sometimes abbreviated as a front surface, a resin layer coating on the front surface will be sometimes abbreviated as a front resin layer, a side opposite thereto will be sometimes abbreviated as a reverse side, and a resin layer formed on the reverse surface will be sometimes abbreviated as a reverse resin layer). The above resin-coated paper is obtained by a series of steps of casting onto a running base paper a film of polyethylene resin composition extruded through a slit die of a melting extruder; pressing them in a nip of a pressing roll and a cooling roll to bond them; cooling the resultant laminate; and then peeling it from the roll. In this case, for producing a resin-coated paper for an imaging material for glossy use, there is used a cooling roll which has a mirror surface, a gloss surface or a finely roughened surface and has an excellent smoothness. In this manner, the front resin layer in a molten state is brought into contact with the cooling roll having an excellent smoothness under pressure. Therefore, the front resin layer could be processed so as to have a surface having an excellent smoothness, and an imaging material using the above resin-coated paper as a support and a print thereon-could have a visually high gloss. However, concerning an imaging material using an actually produced resin-coated paper as a support and a print thereon, it has not been possible to obtain any product having a high-gloss appearance. Concerning a photographic paper using a resin-coated paper in particular, it has not been possible to obtain a photographic paper and a print thereon having a sufficiently high-gloss appearance.
The present inventors have therefore made studies for factors of the high-gloss appearance of imaging materials and their prints. As a result, the gloss appearance is affected by various factors such as a resin-coated paper as a support, an image-forming layer and an image-forming method such as development, while it has been found that the gloss appearance is also greatly affected by the factor of a resin-coated paper as a support. The present inventors have therefore made studies on the factor of a resin-coated paper which affects the appearance of gloss. As a result, it has been found that the gloss appearance not only depends upon the factor of a resin layer but also depends upon a variety of factors including factors of the kind and properties of a base paper composed mainly of a natural pulp such as the kind of a natural pulp and a fiber length, conditions of a paper material slurry such as additives for paper, contained in a paper material slurry, paper-making conditions such as a paper-making speed, a bulk density increasing press conditions and machine calender conditions, post-treatment conditions such as size press and tub size press, and further, the surface roughness of a base paper. It has been also found that as the thickness of a front resin layer of a resin-coated paper decreases, the gloss appearance of an imaging material using the above resin-coated paper as a support and a print thereon decreases, and that when the above thickness is 31 μm or less, the above gloss appearance greatly decreases. A photographic material for glossy use is required to give a print having a high gloss appearance, and the problem is that a photographic material which gives a photographic print having a poor gloss appearance is absolutely not suitable for glossy use and has no commercial value.
Second, a resin-coated paper for an imaging material for glossy use is required to have high smoothness. When a base paper is coated with a molten resin by extrusion, however, as the thickness of a front resin layer increases, in particular, when the above thickness is 20 μm or greater or as the speed of production of the resin-coated paper increases, in particular, when the above speed is 200 m/mn or greater, the peeling of the resin-coated paper from a cooling roll is degraded, and a non-uniformity in the form of a lateral height difference in a width direction, called “peel non-uniformity”, occurs on the resin-coated paper. When the above peel non-uniformity occurs, an imaging material using the resin-coated paper as a support and a print thereon cause gloss non-uniformity. The problem is that the gloss appearance further deteriorates and that the commercial value thereof extremely decreases.
Conventionally, there are some methods proposed for overcoming the above problems and some other problems of a support of a resin-coated paper type for an imaging material. For example, there is known a method in which crater-shaped pores which are liable to occur in the front resin layer surface of a photographic support of a resin-coated paper type are prevented or overcome by double layer extrusion coating method by means of co-extrusion coating or consecutive extrusion coating, to provide a photographic support which is free of surface defects and is excellent in smoothness. However, the above method is insufficient for overcoming the above-explained problems, and in particular, it is absolutely insufficient for improving the gloss appearance of an imaging material using a resin-coated paper as a support and a print thereon.
On the other hand, for improving a resin-coated paper in smoothness, there are known methods using a specific pulp such as a pulp having a specific fiber length distribution, a pulp having specific fiber length, width and thickness, a specific conifer pulp or a specific low-density pulp, a base paper having a specific physical property value such as a base paper having a Beck smoothness equivalent to, or greater than, a specific value or a base paper having a surface roughness equivalent to, or smaller than, a specific value. For the same purpose, there is known a method of hot calendering of a base paper or there is known a specific paper-making method such as paper-making with a paper machine having an upper dehydration mechanism, paper-making with a Fourdrinier two-layer paper machine or the bulk density increasing press of a wet paper. However, these methods are still insufficient for overcoming the above problems, and in particular, they are absolutely insufficient for improving the gloss appearance of an image material using a resin-coated paper as a support and a print thereon.
Meanwhile, the most simplest method for improving the smoothness of a resin-coated paper for glossy use is, generally, to increase the thickness of the front resin layer. However, as the thickness of the front resin layer is increased, particularly, when the above thickness is greater than 31 μm, there is caused a problem that a resin-coated paper, an imaging material using the resin-coated paper as a support and its print curl toward an image-forming layer side and are much troublesome to handle, i.e., a problem that the curl resistance is degraded.
Further, a resin-coated paper is improved in smoothness by using a base paper having excellent smoothness as a base paper for the resin-coated paper. However, there is often involved a problem that an imaging material using the above resin-coated paper as a support and its print have a poor stiffness. When an imaging material, a photographic material in particular, has a poor stiffness, there is sometimes caused a problem that the developability, automatic developability in particular, is degraded. Further, a print is manually taken up for its appreciation, and a “panorama” having a large width has a problem that it is difficult to appreciate when it has a poor stiffness. An imaging material and its print are therefore required to have a strong stiffness, while, as a result of studies by the present inventors, it has been found that the stiffness of an imaging material and its print greatly depends upon the strength of stiffness of a resin-coated paper as a support and that the stiffness of the resin-coated paper greatly depends upon the strength of stiffness of a base paper. However, the problem of stiffness of a base paper often has a contradicting relationship with the smoothness of the base paper, and the following inconsistent problems have been found. When the smoothness is good, the stiffness is poor. When the stiffness is sufficient, the smoothness is poor, and as a consequence, the stiffness of an imaging material using the resin-coated paper as a support and its print is poor, or the gloss appearance of the print, a photographic print in particular, is poor.
There are some methods conventionally proposed for overcoming the above problems of a support for an imaging material of a resin-coated paper type. JP-A-61-132949 discloses a method for providing a photographic support of a resin-coated paper type having a high rigidity and a high gloss by a photographic base paper formed of a first coating film composed mainly of a low-density polyethylene and a second coating layer composed of a polymer having a high rigidity modulus. As a polymer having a higher rigidity modulus, the above publication discloses high-density polyethylene (HDPE), polypropylene (PP), polycarbonate (PC), linear low-density polyethylene (LLDPE), polyamides such as nylon 11, nylon 6 and nylon 66, and polyesters such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT). However, the use of the above method is still insufficient for improving the gloss appearance of an imaging material using a resin-coated paper as a support and a print thereon, and there occurs a problem that the curl resistance is degraded. That is, the following problem occurs. When a polymer having a high density is used as a polymer in the second coating layer, and in particular, with an increase in the above density or with an increase in the content of the above polymer in the coating layer, a resin-coated paper, an imaging material using the resin-coated paper as a support and its print show poor curl resistance.
Further, JP-A-7-120868 discloses a method in which at least two water-resistant-resin-coated layers are formed and a water-resistant resin of a layer farthest from a base paper has a higher density than a water-resistant resin of any other layer(s), and JP-A-7-168308 discloses a method in which at least two water-resistant-resin-coated layers are formed and a resin having a specific flexural modulus is used as a water-resistant resin for an outer-most layer, for improving the adhesion between a base paper and the water-resistant resin layers and the property of peeling from a cooling roll, to provide a support for a photographic paper of a resin-coated paper type. However, the above methods are still insufficient for improving the gloss appearance of an imaging material using a resin-coated paper as a support and of a print thereon. Further, there occurs another problem that the resin-coated paper and an imaging material using the resin-coated paper as a support are degraded in curl resistance. That is, the following problem occurs. When a water-resistant resin having a high density is used as a water-resistant resin in a coating layer, and in particular, with an increase in the above density or with an increase in the content of the above water-resistant resin in the coating layer, a resin-coated paper, an imaging material using the resin-coated paper as a support and its print show poor curl resistance.